The present invention relates generally to composite materials, and more particularly to composite materials including machines embedded in polymeric matrix materials, and even more particularly to fluid-filled bodies embedded in polymeric matrix materials that act as damping materials.
Many composite materials have been suggested as an alternative to traditional materials, such as metal or wood. Generally, such materials include fabric or strands of fiber, such as kevlar, carbon or glass, that are impregnated within a binding matrix, such as an epoxy resin. The strands are arranged within the matrix in a predetermined orientation to provide desired physical properties for the material. For example, composite materials are often designed to provide increased rigidity and strength at substantially less weight as compared to traditional materials.
Composite honeycomb materials have also been suggested that include a honeycomb core sandwiched between two skins. The honeycomb material may be formed from plastic, metal, or fiber reinforced plastic, which may also provide enhanced structural properties at substantially less weight as compared to traditional materials. Foam-core structures formed from a variety of plastics or fiber reinforced plastics have also been suggested that may have similar properties to honeycomb materials.
One application for composite materials is as fairings used to secure payloads within launch vehicles or spacecraft. Such vehicles experience substantial vibroacoustic forces during launch, for example, generated by engine exhaust flow or by aerodynamic forces on the vehicle skin. The skin reaction to these forces takes the form of vibrations that are transmitted to items mounted on the skin and may be carried by structural connections to other locations within the vehicle. These forces may cause fatigue or other failures of components within the vehicle, such as electrical units or wiring, which may be mounted to the skin.
Additionally, the skin vibration may be re-radiated into the interior spaces of a vehicle as acoustic energy. This re-radiation may be particularly significant to the payload fairings. The acoustic levels experienced by such fairings have been critical design considerations for controlling vibrational effects on payloads mounted on these fairing surfaces. Traditional techniques for mitigating these effects include surface treatments, blankets, acoustic absorbers, such as resonators, and active noise control schemes.
The radiation of energy through a structure is inversely proportional to its material stiffness and damping losses. Composite materials have been incorporated into fairing designs, because of their high structural stiffness and low overall weight. These designs, however, generally have poor sound blocking qualities and, consequently, blankets or Helmholtz absorbers are often used to provide additional damping, which may negate any weight savings and increase fabrication complexity.
Damping materials have been suggested that include porous materials within which viscous fluids are entrapped, such as that disclosed in U.S. Pat. No. 5,965,249. Viscoelastic materials may be mixed with matrix material at high temperature, and the matrix material subsequently hardened, thereby trapping the viscoelastic materials within pores of the matrix. Thus, this material is similar to a sponge, having random pores therein that are filled with fluid. Such a material, however, may not exhibit substantially uniform physical properties, and may not be designed to respond in a predetermined manner to particular types of forces that may be experienced during use.
Accordingly, composite materials that damp forces, such as vibroacoustic forces, would be considered useful.
The present invention is directed to composite materials, and more particularly to fluid-filled three dimensional bodies or xe2x80x9cmachinesxe2x80x9d that are embedded in polymeric matrix materials that may act as damping materials.
In accordance with one aspect of the present invention, a composite material is provided that includes a matrix material and a first body disposed within the matrix material. The first body includes a first internal space configured to change volume when the composite material is subjected to a predetermined force. A fluid is provided within the first internal space, whereby fluid may flow within, into, and/or out of the first internal space when the predetermined force changes the volume of the internal space.
Preferably, the fluid is a viscous fluid having a predetermined viscosity for dissipating energy when the predetermined force changes the volume of the first internal space. More preferably, the fluid is substantially incompressible.
In one embodiment, a reservoir is provided for receiving fluid from or adding fluid into the first internal space in response to the predetermined force. Alternatively, a second body may be disposed within the matrix material, the second body comprising a second internal space configured to change volume when the composite material is subjected to a predetermined force, the second internal space communicating with the first internal space. Preferably, the first internal space is configured to increase in volume and the second internal space is configured to decrease in volume in response to the predetermined force, thereby providing a substantially closed system.
In a further alternative, the fluid within the internal space may include one or more bubbles of compressible fluid with the remainder of the internal space being filled with an incompressible fluid. For example, a compressible bubble may be provided that extends across the cross-section of the internal space, e.g., due to surface tension, thereby dividing the internal space into two regions of incompressible fluid separated by a compressible xe2x80x9creservoir.xe2x80x9d The bubble may act as a reservoir, because it may expand or contract as the volume of the internal space changes when the body changes shape to accommodate the incompressible fluid. Alternatively, a plurality of bubbles may be suspended or otherwise dissolved within the incompressible fluid that may act as reservoirs in a similar manner.
In one embodiment, the body is an elongate tubular member defining a longitudinal axis and a cross-section. The cross-section is configured to change shape in response to the predetermined force, thereby changing the volume of the internal space. A fluid within the internal space has a predetermined viscosity, whereby, as the volume of the internal space changes in response to the predetermined force, the fluid moves within the internal space, thereby damping the predetermined force.
More preferably, an array of elongate tubular members is arranged in a predetermined configuration within the matrix material, for example, in a plane. Each tubular member preferably includes a pair of opposing planar portions arranged substantially parallel to the plane. The opposing planar portions may move relative to one another within the matrix material to change the volume of the internal space. Each tubular member also includes a pair of connecting portions extending between the opposing planar portions, the connecting portions limiting relative movement of the opposing planar portions within the matrix material in a predetermined manner.
In one embodiment, the connecting portions may be transverse portions extending between the opposing planar portions, thereby defining a generally xe2x80x9czxe2x80x9d shaped cross-section. In another embodiment, the connecting portions may be curved portions extending between the opposing planar portions, thereby defining at least one of an hourglass cross-section and an apple cross-section.
In another embodiment, the tubular members may be generally cylindrical members arranged substantially parallel to the longitudinal axis. Preferably, some of the cylindrical members maintain substantially constant cross-sections when subjected to a tensile force directed substantially parallel to the longitudinal axis, while other cylindrical members decrease in cross-section when subjected to the tensile force.
Thus, a composite material in accordance with the present invention may be used for damping energy within a composite material. Preferably, pairs of complementary bodies are disposed within a matrix material, the bodies being filled with a viscous fluid. The composite material may be subjected to a predetermined force, e.g., a vibrational force, such that the bodies change volume, thereby causing the fluid to move into or out of the bodies to dampen displacement of the composite material due to the predetermined force. Preferably, due to the predetermined force, one of the bodies increases in volume while the other body decreases in volume, thereby causing viscous fluid to flow between the bodies to damp energy from the predetermined force. The predetermined force that may be damped may include a shear force directed substantially parallel to the plane, a compressive/tensile force directed substantially transverse to the plane, and/or a compressive/tensile force directed substantially parallel to the plane.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.